1. Field of the Invention
The present invention relates to a control apparatus for a hydraulically operated vehicular transmission having a plurality of speed stages to be established by a selective operation of a plurality of hydraulic engaging elements. In this specification, the term "vehicular transmission" means a transmission for a vehicle such as a motor vehicle.
2. Description of the Related Art
With regard to this kind of control apparatus, there has hitherto been proposed the following one in Japanese Patent Application No. 253633/1996. Namely, the hydraulic pressure of a hydraulic engaging element on an engaging side to be engaged at the time of speed changing (engaging pressure) and the hydraulic pressure of a hydraulic engaging element on a disengaging side to be disengaged at the time of speed changing (disengaging pressure) are electronically variably controlled by using valve means including solenoid proportional valves, or the like. An input and output speed ratio (rotational speed of output shaft/rotational speed of input shaft) of a transmission is detected to thereby discriminate the state of development of speed changing. Based on the result of the discrimination, the valve means is controlled to change the engaging pressure and the disengaging pressure so that a smooth speed changing without speed change shocks can be attained.
In this kind of control apparatus, there is also known an apparatus having the following arrangement. Namely, the hydraulic pressure control within a limited hydraulic pressure range which is lower than the line pressure to be supplied to the hydraulic engaging element on the engaging side after completion of speed changing is taken care of, or performed, by the valve means. The resolution of the hydraulic pressure control is thus increased so that a delicate control of the engaging pressure and the disengaging pressure in the transition period of speed changing can be controlled with greater accuracy. In this control apparatus, the engaging pressure is increased or boosted to the line pressure after the input and output speed ratio of the transmission has been changed to a predetermined value which serves as a basis for judging whether or not the speed changing has been completed. The speed changing is thereby completed.
When the vehicle has been used for a long period of time, the torque transmission capacity sometimes decreases due to deterioration, or the like, in the friction surface of the hydraulic engaging element. If the torque transmission capacity of the hydraulic engaging element decreases in this manner, the hydraulic engaging element on the engaging side sometimes gives rise to slipping and the speed changing does not proceed any further, even if the hydraulic pressure is increased to a value near the upper limit in the range of the hydraulic pressure to be handled by the valve means.
In this case, the following method is considered. Namely, the lapse of time from the time of starting the speed changing is measured. When this lapse of time has reached a predetermined time, the engaging pressure is increased to the line pressure based on a judgement that there exists an abnormality. The speed changing is thus completed. Here, the transmission torque at the time of speed changing varies with the throttle opening of the engine, or the like. When the transmission torque is large, the progress of speed changing at the initial stage of speed changing is delayed. Even if the hydraulic engaging element is normal, the time required from the starting of speed changing to the completion thereof becomes long. The above-described predetermined time must be set longer than the upper limit time in the range of change of time for speed changing due to the change in the transmission torque. Therefore, in the above-described system, when the speed changing has ceased to proceed or progress due to an abnormality in the hydraulic engaging element, it takes time for the engaging pressure to be boosted to the line pressure. This results in a delay in the completion of speed changing with a consequent poor drivability.
In view of the above point, the present invention has an object of providing a control apparatus for a hydraulically operated vehicular transmission, in which the speed changing can be quickly completed even when the speed changing has ceased to proceed due to an abnormality in the hydraulic engaging element.